Being lean and light is an advantage in the sport of triathlon. If proof of this obvious fact was ever needed, it came in 2011, when Swiss researchers measured the body weight and body fat percentage of 184 age-group triathletes right before they participated in an Ironman event. After the race, the researchers compared the athletes’ body measurements against their Ironman swim, bike and run times, as well as their overall finish times. Body weight was found to have a statistically moderate effect on total race time, with lighter athletes tending to reach the finish line quicker. Body fat percentage had a large effect on total race time and a moderate effect (bordering on large) on swim, bike and run splits.

Not surprisingly, extra body weight and body fat impacted running performance more negatively than swimming or cycling performance because gravity affects running more than it affects swimming and cycling. This is why elite runners are typically smaller than elite swimmers and cyclists.

In a nutshell, this study confirmed what we already knew: It pays to be lean and light in triathlon. While some athletes are naturally skinnier than others, each athlete has an ideal racing weight that is attained when he or she has gotten rid of as much excess body fat as possible through healthy nutrition and proper training.

How much should you weigh?

Your ideal racing weight is determined primarily by your body fat level. There’s not much you can do about the other sources of mass in your body: bone, muscle, water, etc. No matter how well you train or how carefully you eat, most of that weight will stay put. It’s excess body fat that accounts for the difference between a triathlete’s current weight and his or her ideal racing weight in most instances, and thus it’s fat mass that must be lost to attain the ideal racing weight.

So, what is your ideal racing weight? The only way to definitively determine it is functionally—in other words, by actually attaining it. By definition, your optimal racing weight and optimal body fat percentage are the numbers that are associated with your highest level of fitness. Therefore you’ll know your ideal racing weight with certainty only when you get in the best shape of your life and then weigh yourself and measure your body composition on the day of a breakthrough race.

In the meantime, however, you can create a reasonably accurate estimate of your optimal racing weight to use as a target for your weight-management efforts. Given the fact that body fat is the primary determinant of ideal racing weight, the best way to estimate it is to calculate how much you will weigh after you’ve reduced your body fat percentage to the optimal level for you. Optimal body fat percentage is not the same for everyone. There are many factors that affect how lean each individual triathlete can become. These factors include gender, age, genetics and history of being overweight. However, even triathletes who have all of these factors working against them can get fairly lean.

The following table presents optimal body fat percentage ranges for triathlete of different genders and age groups. Most can expect to get their body fat percentage down within the optimal range associated with their gender and age group through proper training and diet.

It is likely that you will be able to reach the lower limit of your ideal range only if you typically lose weight fairly easily, you have never been seriously overweight and you are willing and able to maintain a high training volume. If your current body fat percentage is well above your optimal range, you should aim only to reach the upper limit of that range initially through increased training and improvements in diet.

Estimating the body fat percentage you can realistically expect to attain at your peak fitness level is not an exact science. Just use common sense and the considerations above to make an educated guess for yourself.

The next step in determining your racing weight is to calculate how much fat weight you will lose in the process of getting down to your goal body fat percentage. Let’s look at how to do this with an example.

Imagine you are a 38-year-old female who currently weighs 140 pounds and has 22 percent body fat. Your diet is already very clean, but you have always had great difficulty shedding excess body fat. Therefore you decide to make a conservative racing weight estimate, at least to start with. Your initial goal is to get down to 17 percent body fat (the upper limit of your ideal range) through improved training and diet. Here’s how to calculate how much you will weigh when you get your body fat percentage down to that level:

Getting leaner the right way

According to a scientific survey conducted by researchers at Montana State University, more than half of endurance athletes believe they are above their optimal racing weight at any given time. This means that, if you’re like most triathletes, you are looking to shed some excess body fat before your next race. There are right and wrong ways to pursue this objective. Here are the key do’s and don’ts of performance weight management: